I think you have to be careful, but during 'Make a WillMonth' our hospice has reached agreement with most of the local solicitors in the town that they will provide free will making advice if a donation to the hospice is included in it. So these are the usual firms, just providing the service for free. If you have a particular charity you were thinking of leaving money to, then why not ask them about 'Make a Will Month'?
It is October this year, some charities use freewillsmonth.org.uk/ but feel free to ask your local ones if more appropriate?